An apparatus to measure the thermal conductivity of insulation panels at sub-ambient temperature

A single-sided guarded-plate apparatus has been developed to measure the thermal conductivity of insulation panels of sub-meter size at sub-ambient temperatures ranging from 250 to 300 K. This apparatus allows thermal conductivity measurements to be performed at large temperature differences simulating the actual operating conditions in an application and can accommodate panels of various thickness. The cold plate in the apparatus is cooled with a flow thermostat. The reduction in performance due to the heat conductance along the edge of a vacuum insulation panel is significant at small panel sizes and is detrimental to the end application. The effective thermal conductivity of square vacuum insulation panels of several thickness and area is measured, and is observed that the square panels of side 10 and 20 cm have about twice higher thermal conductivity than their larger counterparts due to the heat leak along the edges of these panels.